1. Field of the Invention
The present invention relates to a method of treating the surface of an organic insulating film and a method of fabricating a thin film transistor using the same, and more particularly, to a method for treating the surface of an organic insulating film for improving the adhesive strength between the organic insulating film and a transparent electrode and a method of fabricating a thin film transistor substrate using the same.
2. Discussion of the Related Art
A liquid crystal display device controls the light transmissivity of liquid crystal using an electric field to display a picture. To this end, the liquid crystal display device includes a liquid crystal display panel having liquid crystal cells arranged in a matrix configuration and a driving circuit for driving the liquid crystal display panel.
In the liquid crystal display panel, gate lines and data lines are arranged respectively crossing each other. The liquid crystal cells are positioned at each area where the gate lines cross the data lines. The liquid crystal display panel is provided with a common electrode and pixel electrodes for applying an electric field to each of liquid crystal cells. Each pixel electrode is connected to one of the data lines via a thin film transistor (hereafter TFT) as a switching device.
The gate terminal of the thin film transistor is connected to a respective one of the gate lines allowing a pixel voltage signal to be applied to the pixel electrodes for one line. The driving circuit includes a gate driver for driving the gate lines, a data driver for driving the data lines, and a common voltage generator for driving the common electrode. The gate driver sequentially applies a scanning signal. Namely, a gate signal is applied sequentially to the gate lines to drive the liquid crystal cells on the liquid crystal display panel one line by one line. The data driver applies a data voltage signal to each of the data lines whenever the gate signal is applied to any one of the gate lines. The common voltage generator applies a common voltage signal to the common electrode.
Accordingly, the LCD controls the light transmissivity by changing the arrangement state of the liquid crystal between the pixel electrode and the common electrode in accordance with the pixel voltage signal for each liquid crystal cell, thereby displaying a picture.
FIG. 1 is an electrode arrangement plan of a thin film transistor substrate included in a general liquid crystal display device. As shown, the liquid crystal display device includes a thin firm transistor (TFT) substrate having a data line 2 and a gate line 4 arranged respectively crossing each other.
A TFT 10 is provided at the intersection of the data line 2 and the gate line 4. A pixel electrode 20, which is connected to a drain electrode 18 of the TFT 10 through a first contact hole 19, is provided in a cell area. The pixel electrode 20 is connected to the data line 2 via the drain electrode 18, an active layer 14 and a source electrode 16 of the TFT 10. A gate electrode 12 of the TFT 10 is connected to the gate line 4. The TFT 10 corresponds to a gate signal supplied to the gate line 4 to have a pixel voltage, which is applied to the data line 2, charged the pixel electrode 20 with and sustained.
A potential difference is generated between the pixel electrode 20 and a common electrode (not shown) formed on a upper substrate (not shown) because of the charged pixel voltage. By this potential difference, the liquid crystal positioned between the TFT substrate and the upper substrate, rotates as a result of a dielectric anisotropy, thereby transmitting to the upper substrate a light incident via the pixel electrode 20 from a light source (not shown).
The pixel electrode 20 is formed to overlap with the previous gate line 4 so that a storage capacitor 24 is formed for steadying the pixel voltage charged or the pixel electrode 20. To increase the capacitance of the storage capacitor 24, an overlapping storage electrode 21 is further included and has the gate line 4 and a gate insulating layer in between them.
The storage electrode 21 is connected to the pixel electrode 20 through a second contact hole 23 penetrating a protective film (not shown). The data line 2 is connected to a data driver (not shown) through a data pad portion 13, and the gate line 4 is connected to a gate driver (not shown) through a gate pad portion 11. The gate pad portion 11 includes a gate pad electrode 6 extended from the gate line 4 and a protective electrode 22 connected to the gate pad electrode 6 through a third contact hole 5 penetrating a gate insulating film (not shown) and a protective film (not shown). The data pad portion 13 includes a data pad electrode 8 extended from the data line 2 and a protective electrode 22 connected to the data pad electrode 8 through a fourth contact hole 7 penetrating the protective film.
To describe in detail the fabricating method of the TFT substrate with such a structure, it is illustrated in FIGS. 2A to 2C.
FIG. 2A provides a cross-sectional view illustrating a first step of a conventional fabricating method of the thin film transistor substrate shown in FIG. 1. As shown in this drawing, there are formed gate patterns, source/drain patterns and a thin film transistor TFT 10 on a lower substrate 1. Firstly, a gate metal layer is entirely deposited on the lower substrate 1 by a deposition method such as sputtering technique or the like. Chromium (Cr), molybdenum (Mo), metal of aluminum system or the like may be used as the gate metal layer having a single layer structure or a double layer structure. The gate metal layer is patterned on the lower substrate 1 by a photolithography process using a first mask and an etching process to form gate patterns including the gate line 4, the gate electrode 12 and the gate pad electrode 6.
A gate insulating film 3 is entirely formed on the lower substrate 1, where the gate patterns have been formed by a deposition method such as plasma enhanced chemical vapor deposition (PECVD) or the like. The material for the gate insulating film 3 may be an insulating material such as silicon oxide (SiOx), silicon nitride (SiNx) or the like.
A semiconductor layer and a semiconductor layer doped with impurities are continuously deposited on the gate insulating film 3 by a deposition method such as a PECVD technique or the like. Amorphous silicon or polycrystalline silicon may be used for the semiconductor layer. Subsequently, the semiconductor layer and the semiconductor layer doped with impurities are patterned by the photolithography using a second mask and the etching process to form an active layer 14 and an ohmic contact layer 15.
A source/drain metal is entirely deposited by the deposition process such as the sputtering technique on the gate insulating film 3 where the active layer 14 and the ohmic contact layer 15 have been formed. Molybdenum (Mo), titanium, tantalum, molybdenum alloy or the like may be used for the source/drain metal. Subsequently, the source/drain metal is patterned by the photolithography process using a third mask and the etching process to form source/drain patterns such as the data line 2, the source electrode 16, the drain electrode 18, the storage electrode 21, and the data pad electrode 8. Then, the source electrode 16 and the drain electrode 18 are dry-etched using a mask to eliminate the ohmic contact layer 15 between the source electrode 16 and the drain electrode 18.
FIG. 23 is a sectional view illustrating a second step of a conventional fabricating method of the thin film transistor substrate shown in FIG. 1. As shown, an organic insulating film 26 is entirely formed by the process such as spin-coating and the like on the gate insulating film 3 where the source/drain patterns have been formed. For the material of the organic insulating film 26, an organic compound of acrylic system, benzocyclobutene (BCB), perfluorocyclobutane (PFCB) and the like which have small dielectric constant, may be used. The organic insulating film 26 is patterned by the photolithography process using a fourth mask and the etching process to form first through fourth contact holes 19, 23, 7 and 5. Each of the first through fourth contact holes 19, 23, 7 and 5 respectively exposes the drain electrode 18, the storage electrode 21, the data pad electrode 8 and the gate pad electrode 6. Herein, the third contact hole 5, formed at the gate pad portion 11, is formed penetrating through the gate insulating film 3.
FIG. 2C is a sectional view illustrating a third step of a conventional fabricating method of the thin film transistor substrate shown in FIG. 1. As shown, transparent electrode patterns including the pixel electrode 20 and the protective electrode 22 are formed on the organic insulating film 26. A transparent electrode material is entirely deposited on the organic insulating film 26 by a deposition method such as sputtering technique or the like. For the transparent electrode material, indium-tin-oxide (ITO), indium-zinc-oxide (IZO) or indium-tin-zinc-oxide (ITZO), may be used. The transparent electrode material is patterned by the photolithography process using a fifth mask and the etching process to form the pixel electrode 20 and the protective electrode 22 of the pad portions 11 and 13.
The pixel electrode 20 electrically contacts the drain electrode 18 through the first contact hole 19 and the storage electrode 21 through the second contact hole 23. The protective electrode 22 electrically contacts the gate pad electrode 6 and the data pad electrode 8 through the third and the fourth contact holes 5 and 7, respectively. The edge area of the pixel electrode 20 is capable of being formed to overlap with the data line 2 because the organic insulating material having small dielectric constants is adopted for the protective film 26. As a result, the area of the pixel electrode 20 is increased to improve the aperture ratio.
In this way, the aperture ratio can be improved by adopting the organic insulating material for the protective film 26 in the conventional thin film transistor substrate. However, the organic insulating film 26 has a disadvantage of an insufficient adhesive strength to the transparent electrode material because the organic insulating film 26 is formed by the spin-coating technique to achieve a surface smooth. Consequently, problems may occur. For example, etchant may penetrate between the organic insulating film 26 and the transparent electrode layer where the adhesion of the organic insulating film is insufficient during a wet patterning process after deposition of the transparent electrode material on the organic insulating film 26. As a result, critical defects such as a broken wire of the pixel electrode or the like, may occur.